The invention generally relates to the separation and recovery of SF6 from a gas stream comprising CF4 and at least one of air and nitrogen. The invention specifically relates to a method and apparatus for the separation and recovery of SF6 from a gas stream comprising CF4 and at least one of air and nitrogen using a combination of membrane and liquefaction separation techniques.
Sulfur hexafluoride (SF6) is widely used in the electric power distribution industry. In particular, SF6 is used as an insulator or dielectric gas in power distribution equipment such as transformers, switch boxes, gas insulated lines, and substations. Under high voltage conditions, SF6 decomposes into various polar components including HF, F+, SO2, and the like. These by-products degrade the insulating qualities of the gas. As a result, the gas has to be replaced or refined periodically, In addition to the formation of these polar by-products, carbon tetrafluoride (CF4) is also generated during arcing in the presence of carbon containing insulators such as the materials known under the trade designations Teflon(copyright) and Megelit(copyright).
Currently, there are several known methods for purifying and recycling SF6 used as an insulator in electrical equipment. These methods are based upon adsorption and liquefaction. The SF6 polar decomposition by-products are removed by soda lime, activated alumina, or molecular sieves. The refined SF6 is then refilled into the circuit breakers, substations, or transformers.
Due to both technical and environmental concerns, the use of SF6/N2 (or SF6/air) mixtures has been suggested to replace pure SF6 as a gaseous dielectric in the electric power distribution industry. However, there is no known method or apparatus that can economically and efficiently recover and refined SF6 from a gas mixture containing N2 or air and CF4. Thus, there is a need in the industry for an economical and efficient method and apparatus for the capture and recycle of SF6/N2 mixture containing CF4 and the polar by-products. The present invention is intended to address this need in the art.
Briefly, in one aspect, the present invention relates to a method for the paration of SF6 from a gas mixture consisting essentially of SF6, CF4, and N2.
The method comprises the steps of:
(a) contacting a gas mixture consisting essentially of SF6, CF4, and N2 with a membrane at conditions effective to obtain a permeate stream rich in N 2 and a retentate stream rich in SF6 and CF4; and
(b) liquefying the retentate stream at conditions effective to obtain liquid SF6 and gaseous CF4.
In another aspect, the present invention relates to an apparatus for the separation of SF6 from a gas mixture consisting essentially of SF6, CF4, and N2.
The apparatus comprises:
(a) at least one membrane separation unit which permeates N2 faster than SF6 and CF4; and
(b) means for liquefying a retentate stream comprising SF6 and CF4 from the at least one membrane separation unit to form liquid SF6 and gaseous CF4.